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Register a new userStellar populations and Star Formation History of the Metal-Poor Dwarf Galaxy DDO 68
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We present the star formation history of the extremely metal-poor dwarf galaxy DDO 68, based on our photometry with the Advanced Camera for Surveys. With a metallicity of only $12+log(O/H)=7.15$ and a very isolated location, DDO 68 is one of the most metal-poor galaxies known. It has been argued that DDO 68 is a young system that started forming stars only $sim 0.15$ Gyr ago. Our data provide a deep and uncontaminated optical color-magnitude diagram that allows us to disprove this hypothesis, since we find a population of at least $sim 1$ Gyr old stars. The star formation activity has been fairly continuous over all the look-back time. The current rate is quite low, and the highest activity occurred between 10 and 100 Myr ago. The average star formation rate over the whole Hubble time is $simeq 0.01$ M$_{odot}$ yr$^{-1}$, corresponding to a total astrated mass of $simeq 1.3 times 10^8$ M$_{odot}$. Our photometry allows us to infer the distance from the tip of the red giant branch, $D = 12.08 pm 0.67$ Mpc; however, to let our synthetic color-magnitude diagram reproduce the observed ones we need a slightly higher distance, $D=12.65$ Mpc, or $(m-M)_0 = 30.51$, still inside the errors of the previous determination, and we adopt the latter. DDO 68 shows a very interesting and complex history, with its quite disturbed shape and a long Tail probably due to tidal interactions. The star formation history of the Tail differs from that of the main body mainly for an enhanced activity at recent epochs, likely triggered by the interaction.
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We present chemical abundances and radial velocities of six HII regions in the extremely metal-poor star-forming dwarf galaxy DDO 68. They are derived from deep spectra in the wavelength range 3500 - 10,000 {AA}, acquired with the Multi Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT). In the three regions where the [O III]$lambda$4363 {AA} line was detected, we inferred the abundance of He, N, O, Ne, Ar, and S through the direct method. We also derived the oxygen abundances of all the six regions adopting indirect method calibrations. We confirm that DDO 68 is an extremely metal-poor galaxy, and a strong outlier in the luminosity - metallicity relation defined by star-forming galaxies. With the direct-method we find indeed an oxygen abundance of 12+log(O/H)=7.14$pm$0.07 in the northernmost region of the galaxy and, although with large uncertainties, an even lower 12+log(O/H)=6.96$pm$0.09 in the tail. This is, at face value, the most metal-poor direct abundance detection of any galaxy known. We derive a radial oxygen gradient of -0.06$pm$0.03 dex/kpc (or -0.30 dex $R_{25}^{-1}$) with the direct method, and a steeper gradient of -0.12$pm$0.03 dex/kpc (or -0.59 dex $R_{25}^{-1}$) from the indirect method. For the $alpha$-element to oxygen ratios we obtain values in agreement with those found in other metal-poor star-forming dwarfs. For nitrogen, instead, we infer much higher values, leading to log(N/O)$sim-1.4$, at variance with the suggested existence of a tight plateau at $-1.6$ in extremely metal poor dwarfs. The derived helium mass fraction ranges from Y=0.240$pm$0.005 to Y=0.25$pm$0.02, compatible with standard big bang nucleosynthesis. Finally, we measured HII region radial velocities in the range 479$-$522 km/s from the tail to the head of the comet, consistent with the rotation derived in the HI.
We present HI spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only 3% Z$_{odot}$, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-metallicity and luminosity-metallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival HI spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass (M$_{rm HI}$ $=$ 2.8$times$10$^{7}$ M$_{odot}$), recently star-forming (SFR$_{rm FUV}$ $=$ 1.4$times$10$^{-3}$ M$_{odot}$ yr$^{-1}$, SFR$_{rm Halpha}$ $<$ 7$times$10$^{-5}$ M$_{odot}$ yr$^{-1}$) companion has the same systemic velocity as DDO 68 (V$_{rm sys}$ $=$ 506 km s$^{-1}$; D $=$ 12.74$pm$0.27 Mpc) and is located at a projected distance of 42 kpc. New HI maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness HI gas forms a bridge between these objects.
The paper presents new results of the ongoing study of the unusual Lynx-Cancer void galaxy DDO 68 with record-low-metallicity regions (12+log(O/H) ~7.14) of the current star formation (SF). They include: a) a new spectrum and photometry with the 6-m SAO RAS telescope (BTA) for the Luminous Blue Variable (LBV = DDO68-V1). Photometric data sets are complemented with those based on the Sloan Digital Sky Survey (SDSS) and the Hubble Space Telescope (HST) archive images; b) the analysis of the DDO~68 supergiant shell (SGS) and the prominent smaller H-alpha arcs/shells visible at the HST image coupled with kinematics maps in H-alpha obtained with the Fabry-Perot interferometer (FPI) at the BTA; c) the list of identified at the HST images of about 50 most luminous stars (-9.1 < M_V < -6.0 mag) related to star-forming regions with the known extremely low O/H. This is intended to pave the path for the actual science with the next generation of giant telescopes. We confirm the earlier hints on significant variations of the LBV optical light deriving its amplitude of dV > 3.7~mag for the first time. New data suggest that in 2008--2010 the LBV reached M_V = --10.5 and probably underwent a giant eruption. We argue that the structure of star-forming complexes along the SGS (`Northern Ring) perimeter provides evidence for the sequential induced SF episodes caused by the shell gas instabilities and gravitational collapse. The variability of some DDO~68 luminous extremely metal-poor stars can be monitored with medium-size telescopes at sites with superb seeing.
We present new Hubble Space Telescope (HST) imaging of a stream-like system associated with the dwarf galaxy DDO 68, located in the Lynx-Cancer Void at a distance of D$sim$12.65 Mpc from us. The stream, previously identified in deep Large Binocular Telescope images as a diffuse low surface brightness structure, is resolved into individual stars in the F606W (broad V) and F814W ($sim$I) images acquired with the Wide Field Camera 3. The resulting V, I color-magnitude diagram (CMD) of the resolved stars is dominated by old (age$gtrsim$1-2 Gyr) red giant branch (RGB) stars. From the observed RGB tip, we conclude that the stream is at the same distance as DDO 68, confirming the physical association with it. A synthetic CMD analysis indicates that the large majority of the star formation activity in the stream occurred at epochs earlier than $sim$1 Gyr ago, and that the star formation at epochs more recent than $sim$500 Myr ago is compatible with zero. The total stellar mass of the stream is $sim10^{6} M_{odot}$, about 1/100 of that of DDO~68. This is a striking example of hierarchical merging in action at the dwarf galaxy scales.
We report the discovery of the most metal-poor dwarf star-forming galaxy (SFG) known to date, J0811+4730. This galaxy, at a redshift z=0.04444, has a Sloan Digital Sky Survey (SDSS) g-band absolute magnitude M_g = -15.41 mag. It was selected by inspecting the spectroscopic data base in the Data Release 13 (DR13) of the SDSS. LBT/MODS spectroscopic observations reveal its oxygen abundance to be 12 + log O/H = 6.98 +/- 0.02, the lowest ever observed for a SFG. J0811+4730 strongly deviates from the main-sequence defined by SFGs in the emission-line diagnostic diagrams and the metallicity - luminosity diagram. These differences are caused mainly by the extremely low oxygen abundance in J0811$+$4730, which is ~10 times lower than that in main-sequence SFGs with similar luminosities. By fitting the spectral energy distributions of the SDSS and LBT spectra, we derive a stellar mass of M* = 10^6.24 - 10^6.29 Msun (statistical uncertainties only), and we find that a considerable fraction of the galaxy stellar mass was formed during the most recent burst of star formation.
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